Which of the following variables in the formula describing Poiseuille's law can be altered in the clinical setting to improve Raw?
1. Radius of the airways
2. Viscosity of the inspired gas
3. Flow rate of the inspired gas
4. Temperature of the inspired gas
5. Length of the tracheobronchial tree
a. 1 and 3 c. 2, 3, and 5
b. 1, 4, and 5 d. 1, 2, and 3
ANS: D
Poiseuille's law is represented by the formula Raw = [8 ? gas flow ? gas viscosity ? airway length] ÷ [??? radius4]. Bronchospasm causes a decrease in the radius of the airways. Bronchodilators can reverse that process, thereby decreasing the Raw. High flow rates cause more turbulence and in-creased Raw. During mechanical ventilation, the flow rate of the gas being delivered can be changed by the respiratory therapist. Helium-oxygen (heliox) is used in respiratory care, not be-cause of its viscosity, but because of its low density. An 80:20 mix of heliox is the least-dense gas available for human use, but it has a viscosity higher than room air. So, heliox does not im-prove Raw. It does, however, make its way around any airway obstruction.
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